1. Field of the Invention
The present invention is directed to a boom-holding control device for use in heavy construction equipments and more specifically to a boom-holding control device that prevents inertia-caused in-travel raising movement and gravity-caused in-work lowering movement of a boom in heavy construction equipments such as an excavator and the like.
2. Description of the Related Art
In the event that a hydraulic excavator stops its operations with a boom raised up, the boom may be unintentionally lowered under the action of its own weight or the weight of loads carried by a bucket. For the purpose of avoiding such boom lowering phenomenon, the conventional hydraulic excavator is provided with a boom holding valve.
By the way, during the course of travel of the hydraulic excavator, the boom often tends to be shaken up and down by virtue of the joggling of an excavator body, at which time the boom is raised up little by little owing to the self-making-up action of the boom holding valve. The boom thus raised may sometimes mar the visibility of an operator and may become an obstacle in passing through overhead road structures, in which case the operator should stop driving the excavator in an effort to lower down the boom into the original position. In view of such inconvenience, there has been proposed a boom control device of the type as illustrated in FIG. 1.
As can be seen in FIG. 1, the prior art boom control device includes a main fluid pump P1 and an auxiliary pump P2, each of which generates a pressurized hydraulic flow. The hydraulic flow discharged from the main pump P1 is supplied to hydraulic actuators such as a boom cylinder 13, a travel motor (not shown) and the like under a control of a plurality of control spools, including a boom control spool 11 and a travel control spool 12, incorporated in a control valve 10. If a remote control valve 20 applies a boom-up pilot signal pressure (Pup) to one pressure receiving part, the boom control spool 11 of the control valve 10 is shifted in one direction to allow the hydraulic flow of the main fluid pump P1 to enter the piston-side chamber 13a of the boom cylinder 13 to thereby raise up the boom. To the contrary, if the remote control valve 20 applies a boom-down pilot signal pressure (Pdn) to the other pressure receiving part, the boom control spool 11 of the control valve 10 is shifted in the opposite direction to allow the hydraulic flow of the main fluid pump PI to enter the rod-side chamber 13b of the boom cylinder 13 to thereby lower down the boom.
A boom holding valve 15 is connected to a fluid pressure line 14 that interconnects the boom control spool 11 and the piston-side chamber 13a of the boom cylinder 13. The boom holding valve 15 includes a poppet 17 slidably fitted into an interior space of the valve in such a manner that the poppet 17 can divide the interior space into a front pressure receiving chamber 15a, a lateral pressure receiving chamber 15b and a rear pressure receiving chamber 15c. A spring 16 is provided on the rear side of the poppet 17 to resiliently bias the poppet 17 toward a position where the front pressure receiving chamber 15a is disconnected from the lateral pressure receiving chamber 15b. The front pressure receiving chamber 15a is in communication with the boom control spool 11 via the fluid pressure line 14 and the rear pressure receiving chamber 15c is led to a fluid tank T by way of a boom release valve 19. The lateral pressure receiving chamber 15b is in communication with the rear pressure receiving chamber 15c through a built-in flow passage 17a and also communicates with the piston-side chamber 13a of the boom cylinder 13 via the fluid pressure line 14.
The boom release valve 19 has a pressure receiving part 19a coupled to the remote control valve 20 through a pilot signal line 21 for reception of the boom-down pilot signal pressure Pdn from the remote control valve 20. The pressure receiving part 19a of the boom release valve 19 is also coupled to a travel pedal valve 22 through travel signal lines 23, 24 for reception of a travel signal pressure from the travel pedal valve 22. Reference numeral 25 designates a changeover valve for selectively opening and closing the travel signal line 24 and reference numeral 26 designates a travel selection switch for shifting the position of the changeover valve 25.
If one of the boom-down pilot signal pressure (Pdn) and the travel signal pressure is exerted on the pressure receiving part 19a of the boom release valve 19, the boom release valve 19 moves into a drain position where the rear pressure receiving chamber 15c communicates with the fluid tank T to thereby release the boom from a holding condition. If, however, neither the boom-down pilot signal pressure (Pdn) nor the travel signal pressure is exerted on the pressure receiving part 19a of the boom release valve 19, the boom release valve 19 is returned back to a shutoff position, by the action of a spring 19b, where the rear pressure receiving chamber 15c is disconnected from the fluid tank T to thereby bring the boom into the holding condition.
In this manner, the boom release valve 19 keeps the boom against raising movement while the excavator travels. At this time, however, the boom holding valve 15 remains opened so that the fluid can be drained from the rod-side chamber 13a of the boom cylinder 13 through the fluid pressure line 14. This means that the weight of the boom is supported by a bucket that has been retracted toward and placed on a frontal part of an excavator body. Under that state, if the excavator runs over an irregular ground surface and is shaken by the vibration imparted to the excavator body, the bucket is repeatedly bumped against the excavator body, which applies a great deal of oscillatory shock to a buck cylinder particularly during the course of long distance travel of the excavator. This may cause severe damage to the bucket cylinder. For avoidance of such damage, the operator should periodically raise up the boom during traveling to reduce the load of the boom acting on the bucket. Needless to say, this makes the operator feel cumbersome.